1. Field of the Invention
The present invention relates to a signal processing apparatus and method, and more particularly, for example, to a signal processing apparatus and method for speedily demodulating an OFDM (Orthogonal Frequency Division Multiplexing) signal.
2. Description of the Related Art
OFDM (Orthogonal Frequency Division Multiplexing) has been adopted for terrestrial digital broadcasting and other broadcasting as a data (signal) modulation scheme.
OFDM uses a number of subcarriers orthogonal to each other within the transmission band, performing PSK (Phase Shift Keying), QAM (Quadrature Amplitude Modulation) or other digital modulation in which data is assigned to the amplitude or phase of each subcarrier.
In OFDM, the transmission band is divided with a number of subcarriers, leading to narrow band for one (wave of) subcarrier and slow modulation speed, but total (whole subcarriers) transmission speed remains the same in the existing modulation scheme.
As described above, data is assigned to a plurality of subcarriers in OFDM. As a result, modulation can be achieved by IFFT (Inverse Fast Fourier Transform) computation adapted to perform an inverse Fourier transform. On the other hand, demodulation of the OFDM signal resulting from modulation can be achieved by FFT (Fast Fourier Transform) computation.
Therefore, an OFDM transmitter adapted to transmit an OFDM signal can be configured with a circuit adapted to perform IFFT computation. On the other hand, an OFDM receiver adapted to receive an OFDM signal can be configured with a circuit adapted to perform FFT computation.
Further, OFDM has signal intervals called guard intervals, thus providing improved multipath immunity. Still further, known signals (signals known to the OFDM receiver), i.e., pilot signals, are inserted discretely in the direction of time or frequency in OFDM so that the OFDM receiver uses these signals for synchronization, estimation of the transmission line (channel) characteristic or other purposes.
Thanks to its high multipath immunity, OFDM has been adopted for terrestrial digital broadcasting and other broadcasting systems subject to significant multipath interference. Among the terrestrial digital broadcasting standards adopting OFDM are DVB-T (Digital Video Broadcasting-Terrestrial) and ISDB-T (Integrated Services Digital Broadcasting-Terrestrial).
With OFDM, data is transmitted in units of an OFDM symbol.
FIG. 1 is a diagram illustrating an OFDM symbol.
An OFDM symbol commonly includes an effective symbol and a guard interval. The effective symbol is a signal period during which IFFT is performed during modulation. The guard interval is a copy of part of the latter half of the effective symbol and attached beginning of the effective symbol.
Thus, providing a guard interval beginning of an OFDM symbol provides improved multipath immunity.
It should be noted that a unit called a frame (OFDM transmission frame) is defined to include a plurality of OFDM symbols in the terrestrial digital broadcasting standard that has adopted OFDM so that data is transmitted in units of a frame.
An OFDM receiver adapted to receive such an OFDM signal performs digital orthogonal demodulation of the OFDM signal using a carrier of the same signal.
It should be noted, however, that the OFDM signal carrier used by an OFDM receiver for digital orthogonal demodulation contains some error because this carrier is not the same as that used by an OFDM transmitter adapted to transmit the OFDM signal. That is, the frequency of the OFDM signal carrier used for digital orthogonal demodulation deviates from the center frequency of the OFDM signal (IF (Intermediate Frequency) signal) received by the OFDM receiver.
Therefore, the OFDM receiver estimates the carrier frequency offset that is an error of the OFDM signal carrier used for digital orthogonal demodulation, and performs carrier frequency offset detection adapted to detect the estimated offset and carrier frequency offset correction adapted to correct the OFDM signal (its carrier frequency offset), thus eliminating the offset in accordance with the estimated offset.
FIG. 2 is a block diagram illustrating an example of configuration of an existing OFDM receiver.
A carrier frequency offset correction section 11 is supplied with a baseband time domain OFDM signal (OFDM time domain signal) obtained after the digital orthogonal demodulation of the OFDM signal.
The same section 11 performs the carrier frequency offset correction adapted to correct the OFDM time domain signal (offset thereof) supplied thereto in accordance with the carrier frequency offset correction amount supplied from a carrier frequency offset correction amount estimation section 15 which will be described later.
The carrier frequency offset correction section 11 supplies the OFDM time domain signal subjected to the carrier frequency offset correction to an FFT computation section 12 and time domain carrier frequency offset detection section 13.
The FFT computation section 12 performs FFT computation adapted to Fourier-transform an OFDM time domain signal from the carrier frequency offset correction section 11 into a frequency domain OFDM signal (OFDM frequency domain signal) and supplies the OFDM frequency domain signal obtained from the FFT computation to a frequency domain carrier frequency offset detection section 14.
It should be noted that the OFDM frequency domain signal obtained from the FFT computation section 12 is supplied not only to the frequency domain carrier frequency offset detection section 14 but also to unshown blocks in the subsequent stage adapted to handle equalization, error correction, decoding and other processes.
The time domain carrier frequency offset detection section 13 performs carrier frequency offset detection adapted to detect the estimated carrier frequency offset by estimating the carrier frequency offset of the OFDM time domain signal using the OFDM time domain signal from the carrier frequency offset correction section 11. The time domain carrier frequency offset detection section 13 supplies (feeds back) the estimated carrier frequency offset obtained from the carrier frequency offset detection to the carrier frequency offset correction amount estimation section 15.
The frequency domain carrier frequency offset detection section 14 performs carrier frequency offset detection adapted to detect the estimated carrier frequency offset by estimating the carrier frequency offset of the OFDM frequency domain signal using the OFDM time domain signal from the FFT computation section 12. The same section 14 supplies (feeds back) the estimated carrier frequency offset obtained from the carrier frequency offset detection to the carrier frequency offset correction amount estimation section 15.
The carrier frequency offset correction amount estimation section 15 estimates the (OFDM time domain signal) correction amount adapted to eliminate the carrier frequency offset of the OFDM time domain signal using either or both of the estimated carrier frequency offsets, one from the time domain carrier frequency offset detection section 13 and another from the frequency domain carrier frequency offset detection section 14. The same section 15 supplies the correction amount to the carrier frequency offset correction section 11.
As described above, the same section 11 corrects the OFDM time domain signal supplied thereto (performs carrier frequency offset correction) in accordance with the correction amount from the carrier frequency offset correction amount estimation section 15.
Incidentally, DVB-T2 (second generation European terrestrial digital broadcasting standard) is on its way to being developed.
It should be noted that DVB-T2 is described in the so-called Blue Book (DVB BlueBook A122) (“Frame structure channel coding and modulation for a second generation digital terrestrial television broadcasting system (DVB-T2),” DVB Document A122 June 2008).
In DVB-T2 (the Blue Book thereof), a frame called a T2 frame is defined so that data is transmitted in units of a T2 frame.
A T2 frame contains two preamble signals called P1 and P2. These preamble signals contain information required for demodulation and other processes (such information is signalled).
FIG. 3 is a diagram illustrating a T2 frame format.
A T2 frame contains one P1 OFDM symbol, one or more P2 OFDM symbols, one or more data (Normal) OFDM symbols, and a necessary FC (Frame Closing) OFDM symbol in this order.
Bits S1 and S2 are, for example, signalled in P1.
The bits S1 and S2 indicate the following information, i.e., whether the symbols other than the P1 (P2, data and FC symbols) are transmitted in SISO (Single Input Single Output) or MISO (Multiple Input Single Output) system, the FFT size for performing FFT computation of the symbols other than P1 (number of samples (symbols) (subcarriers) subject to a single FFT computation) and to which of two groups the guard interval length (hereinafter also referred to as a GI length) belongs.
It should be noted that seven different lengths, i.e., 1/128, 1/32, 1/16, 19/256, ⅛, 19/128 and ¼, are defined relative to the effective symbol length in DVB-T2 as the GI length. These seven GI lengths are classified into two groups. The bits S1 and S2 signalled in P1 contain information as to which of the two groups the GI length of the T2 frame belongs to.
Further, six different numbers of symbols (subcarriers) making up a single OFDM symbol, i.e., FFT sizes, are defined. These sizes are 1 K, 2 K, 4 K, 8 K, 16 K and 32 K.
It should be noted, however, that although any of the above six different FFT sizes can be used for OFDM symbols other than P1 OFDM symbol, only 1 K can be used for P1 OFDM symbol.
As for the FFT size and GI length of P2, on the other hand, the same values as those of the OFDM symbols other than P1 and P2, i.e., data (Normal) and FC OFDM symbols, are used.
Here, P1 contains information required for demodulation of P2 such as the transmission system and FFT size. Therefore, P1 must be demodulated to demodulate P2.
L1PRE and L1POST are signalled in P2.
L1PRE contains information required for the OFDM receiver adapted to receive a T2 frame to demodulate L1POST. L1POST contains information required for the OFDM receiver to access the physical layer (layer pipes thereof).
Here, L1PRE contains information including the GI length, the pilot pattern (PP) indicating the pilot signal arrangement that shows in which symbol (subcarrier) the pilot signal, i.e., the known signal, is contained, whether the transmission band is extended to transmit the OFDM signal (BWT_EXT) and the number of OFDM symbols in one T2 frame (NDSYM). These pieces of information are required to demodulate a symbol containing data (including FC).
After obtaining L1PRE and L1POST (information signalled therein), the OFDM receiver can demodulate the symbol of the data (and FC).
It should be noted that although, in FIG. 3, two P2 OFDM symbols are provided in the T2 frame, any of one to sixteen (16) P2 OFDM symbols may be provided in a T2 frame. However, only one P2 OFDM symbol is provided in a T2 frame containing P2 with an FFT size of 16 K or 32 K.
FIG. 4 is a diagram illustrating a P1 OFDM signal.
A P1 OFDM signal has 1 K (=1024) symbols as effective symbols.
The P1 OFDM signal has a cyclic structure that includes B1′, B1, B2 and B2′: B1′ is a signal obtained by frequency-shifting B1 that is part of the effective symbols in the beginning; B1′ is copied before the effective symbols; B2′ is a signal obtained by frequency-shifting B2 that is the remaining effective symbols; and B2′ is copied after the effective symbols.
The P1 OFDM signal has 853 subcarriers as effective subcarriers. In DVB-T2, 384 at predetermined positions of all the 853 subcarriers are assigned information (locations).
According to the DVB-T2 Implementation Guidelines (ETSI TR 102 831: IG), if the OFDM signal transmission band is, for example, 8 MHz, it is possible to perform “coarse” carrier frequency offset estimation in units of a subcarrier spacing spanning a maximum range from −500 kHz to +500 kHz based on the correlation between the above 384 subcarrier locations.
Further, according to the Implementation Guidelines, it is possible, thanks to the cyclic structure of P1 described in FIG. 4, to perform “fine” carrier frequency offset estimation in units of less than a subcarrier spacing in the range from −0.5× subcarrier spacing to +0.5× subcarrier spacing.
Here, DVB-T2 defines that the FFT size of P1 is 1 K samples (symbols) described in FIG. 4.
Further, DVB-T2 defines that if the transmission band is, for example, 8 MHz, the sampling period of P1 with an FFT size of 1 K samples is 7/64 μs.
Therefore, if the transmission band is, for example, 8 MHz, the P1 effective symbol length Tu is 1024× 7/64 μs.
On the other hand, the relationship expressed by the equation D=1/Tu holds between the effective OFDM symbol length (effective symbol length not including the guard interval) Tu [sec] and the OFDM signal subcarrier spacing D [Hz].
Therefore, if the transmission band is, for example, 8 MHz, the P1 subcarrier spacing D is equal to the reciprocal of the effective symbol length Tu=1024× 7/64 μsec or about 8929 Hz.
As described above, because the P1 subcarrier spacing D is about 8929 Hz, the “fine” estimated carrier frequency offset that can be detected using P1 falls within the range from −8929/2 Hz to +8929/2 Hz.
In this case, the capture range using P1, i.e., the range of frequencies over which OFDM signal carriers used for digital orthogonal demodulation can be pulled in through OFDM signal correction in accordance with the “fine” estimated carrier frequency offset obtained from P1 (range of frequencies over which carrier frequency offset correction can be performed) is in the range 8929/2 Hz above and below the inherent subcarrier position on the frequency axis (frequency) (from −8929/2 Hz to +8929/2 Hz).
Here, the i+1th (where i=0, 1, . . . ) subcarrier from the lowest in frequency of a plurality of OFDM signal subcarriers (OFDM symbols) is denoted by a subcarrier c#i. The inherent frequency (position on the frequency axis) of the subcarrier c#i is referred to as a set frequency f#i.
The OFDM receiver detects, by means of the “fine” carrier frequency offset estimation using P1, the difference between the frequency of the subcarrier c#i of the OFDM signal and a set frequency f#i′ closest to that frequency as a “fine” estimated carrier frequency offset.
Then, the carrier frequency offset correction is performed to correct the OFDM signal in accordance with the “fine” estimated carrier frequency offset so that the frequency of the subcarrier c#i agrees with the set frequency f#i′ closest to that frequency.
Further, the OFDM receiver detects, by means of the “coarse” carrier frequency offset estimation using P1, the difference between the frequency of the subcarrier c#i of the OFDM signal and the set frequency f#i of the subcarrier c#i as a “coarse” estimated carrier frequency offset in units of a subcarrier spacing.
Then, the carrier frequency offset correction is performed to correct the OFDM signal in accordance with the “coarse” estimated carrier frequency offset so that the frequency of the subcarrier c#i agrees with the set frequency f#i of the subcarrier c#i.
Here, the carrier frequency offset correction performed in accordance with the “fine” estimated carrier frequency offset is referred to as a “fine” carrier frequency offset correction, and that performed in accordance with the “coarse” estimated carrier frequency offset as a “coarse” carrier frequency offset correction.